1. Field of the Invention
The present invention relates generally to a laser exposure system for scanning and recording images, and more particularly to a laser exposure apparatus for scanning and recording images by the use of optically modulated laser beams.
2. Description of the Prior Art
It is known in the art to record images by the use of optically modulated laser beams, a laser plotter and color scanner. The laser plotter shown in FIG. 12 is especially well known and widely used. This plotter includes a console 21, a data processor 23 which processes the data input by the console 21 and magnetic tape, a converter 22 for converting the processed data into signals, and a recorder 24 for scanning and recording the images on a photo-sensitive sheet running on a cylinder 25 in response to the signals transmitted from the converter 22. Laser plotters of this kind can record master patterns, such as printed circuit boards, at high speed and high precision.
The recorder 24 is normally provided with a laser exposure device 30 shown in FIG. 13 so as to facilitate the recording of desired patterns. One example of such a plotter is disclosed in Japan Patent Publication (unexamined) No. 60-169820 by this inventor. According to this prior art a laser beam B.sub.11, emitted from a laser source 31, is split into parallel beams B.sub.12 by means of splitter 32, which beams are individually modulated by a multi-channel modulator to become modulated laser beams B.sub.13. The modulated laser beams B.sub.13 are directed to an optical system 34 including a zoom lens 35 and an image-forming lens 36 by a first mirror 37 and a second mirror 38 so as to reduce the beam pitches as desired and form a image 14 on the screen (G).
In the known laser exposure device 30, the laser beams B.sub.13 modulated by the multi-channel optical modulator 33 unavoidably have a diffraction angle 2 .theta., that is, two times as large an angle as the Bragg angle .theta. of the modulator 33. For this reason the first mirror 37 (FIG. 7) and the second mirror 38 are positioned in the direction of diffraction of the laser beams B.sub.4 so that the laser beams B.sub.4 reflecting off of the second mirror 38 may be parallel to the reference plane (S) which is flush with the foundation plane of the laser exposure device with respect to the sub-scanning direction. The first and second mirrors 37 and 38 are positioned in such a manner that the angle of incline of the laser beams B.sub.4 impringing on the first mirror 37 and the angle of incline of the laser beams B.sub.4 " reflected off of the second mirror 38 are complementary for the angle of decline of the laser beam B.sub.4 ' between the first and second mirrors 37, 38. In other words, the angle of propagation of the laser beam is compensated for by an amount .theta. with respect to the reference plane (S) each time the laser beam reflects off a consecutive mirrors 37, 38. FIG. 8 shows the paths taken by the laser beams when the Bragg angle of the modulator is .theta..
The known laser exposure device 30 described above is advantageous in that the laser exposure is effected by adjusting the pitch of the beams forming images in optical dots on the screen (G); nevertheless, the following disadvantages result:
1. When a desired image is formed by reducing the pitches of the beams utilizing the image-forming optical system 34, the main path 55 (FIG. 11) of each laser beam is focused through the lens 36 so as to determine the width (W.sub.1) of the desired image at the image-forming position (Q). This is likely to cause a deviation .DELTA.D between the position (Q) and the recording position (G), thereby varying the width (W.sub.3) of the image depicted by the optical dots in a stepless manner formed by the laser beams.
When a zoom lens is used to adjust the pitch of the beams in a stepless manner, difficulties are likely to arise when photosensitive sheets 26 of different thicknesses are used, because they vary the deviations .DELTA.D.
2. As a practical matter it is difficult to ensure that all the multi-channel optical modulators used have an equal Bragg angle, thereby causing the laser beams B" reflecting off of the second mirror to be inclined with respect to the reference plane in the sub-scanning direction. FIGS. 9 and 10 show images fixed on the photosensitive sheets 26 through laser exposure, each being from the same main scanning position. FIG. 10 shows those obtained when the laser beam B.sub.4 " are parallel to the reference plane (S), in which it will be appreciated that at the first fixed images 101 and the second fixed images 102 are exactly aligned. In contrast, when the laser beams B.sub.4 " are not parallel to each other the images 101 and 102 are misaligned as shown in FIG. 9. This results in poor image quality.
The Bragg angle of a multi-channel optical modulator is normally a very small angle, such as a few milliradians in order, and a possible error is so small (for example one-tenth or far less of a radian thereof) that an incline of laser beams due to such small error in the Bragg angle is usually ignored. However when a high degree of accuracy is required even such a small error cannot be neglected.